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Resim Yükleyin
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Ýlgili Makaleler
Star Formation in the Era of the Three Great Observatories
This paper summarizes contributions and suggestions as presented at theChandra Workshop Star Formation in the Era of Three Great Observatoriesconducted in July 2005. One of the declared goals of the workshop was toraise recognition within the star formation research community about thesensible future utilization of the space observatories Spitzer, Hubble,and Chandra in their remaining years of operation to tackle imminentquestions of our understanding of stellar formation and the earlyevolution of stars. A white paper was generated to support thecontinuous and simultaneous usage of observatory time for star formationresearch. The contents of this paper have been presented and discussedat several other meetings during the course of 2005 and January 2006.
| Mass loss and orbital period decrease in detached chromospherically active binaries
The secular evolution of the orbital angular momentum (OAM), thesystemic mass (M=M1+M2) and the orbital period of114 chromospherically active binaries (CABs) were investigated afterdetermining the kinematical ages of the subsamples which were setaccording to OAM bins. OAMs, systemic masses and orbital periods wereshown to be decreasing by the kinematical ages. The first-orderdecreasing rates of OAM, systemic mass and orbital period have beendetermined as per systemic OAM, per systemic mass and per orbitalperiod, respectively, from the kinematical ages. The ratio of d logJ/dlogM= 2.68, which were derived from the kinematics of the presentsample, implies that there must be a mechanism which amplifies theangular momentum loss (AML) times in comparison to isotropic AML ofhypothetical isotropic wind from the components. It has been shown thatsimple isotropic mass loss from the surface of a component or bothcomponents would increase the orbital period.
| A Comparative Study of Flaring Loops in Active Stars
Dynamo activity in stars of different types is expected to generatemagnetic fields with different characteristics. As a result, adifferential study of the characteristics of magnetic loops in a broadsample of stars may yield information about dynamo systematics. In theabsence of direct imaging, certain physical parameters of a stellarmagnetic loop can be extracted if a flare occurs in that loop. In thispaper we employ a simple nonhydrodynamic approach introduced by Haisch,to analyze a homogeneous sample of all of the flares we could identifyin the EUVE DS database: a total of 134 flares that occurred on 44 starsranging in spectral type from F to M and in luminosity class from V toIII. All of the flare light curves that have been used in the presentstudy were obtained by a single instrument (EUVE DS). For each flare, wehave applied Haisch's simplified approach (HSA) in order to determineloop length, temperature, electron density, and magnetic field. For eachof our target stars, a literature survey has been performed to determinequantitatively the extent to which our results are consistent withindependent studies. The results obtained by HSA are found to be wellsupported by results obtained by other methods. Our survey suggeststhat, on the main sequence, short loops (with lengths<=0.5R*) may be found in stars of all classes, while thelargest loops (with lengths up to 2R*) appear to be confinedto M dwarfs. Based on EUVE data, the transition from small to largeloops on the main sequence appears to occur between spectral types K2and M0. We discuss the implications of this result for dynamo theories.
| On the Temperature-Emission Measure Distribution in Stellar Coronae
Strong peaks in the emission measure-temperature (EM-T ) distributionsin the coronae of some binary stars are associated with the presence ofhot (107 K), dense (up to 1013 cm -3)plasma. These peaks are very reminiscent of those predicted to arise inan impulsively heated solar corona. A coronal model comprised of manyimpulsively heated strands is adapted to stellar parameters. It is shownthat the properties of the EM-T distribution can be accounted for ingeneral terms provided the emission comes from many very small loops(length under 103 km) with intense magnetic fields (1 kG)distributed across part of the surface of the star. The heating requiresevents that generally dissipate between 1026 and 1028 ergs, which is in the range of solar microflares. This impliesthat such stars must be capable of generating regions of localizedintense magnetic fields.
| A Large Millimeter Flare on the RS CVn Binary σ Geminorum
The flaring behavior of active RS CVn binaries has been studiedextensively at centimeter wavelengths, but very little has beenpublished concerning RS CVn flares in the millimeter region, where thenonthermal gyrosynchrotron emission is much more likely to be opticallythin. We present 3 mm (99 GHz) observations of a large flare on theactive binary σ Gem using the Owens Valley Millimeter Array. Two 3hr observations in 2004 February showed only 3 σ upper limits of 3mJy, while the third observation on 2004 April 22 showed a large flareoutburst that filled the 6 hr observation and reached a peak fluxdensity of 90 mJy. The strongest brightening was preceded by at least a4 hr period of significantly elevated emission at 20-40 mJy. Theseobservations demonstrate the feasibility of detailed multifrequencystudies of nonthermal electron distributions in active star coronae withthe next generation of millimeter and centimeter radio arrays.
| MERLIN Astrometry of 11 Radio Stars
We report accurate positions in the International Celestial ReferenceFrame (ICRF) for 11 radio stars. Observations were made using theMulti-Element Radio Linked Interferometer Network at a radio frequencyof 5 GHz. The positions are estimated to be accurate at the 5 mas level.Positions were obtained directly in the ICRF by phase referencing theradio stars to ICRF quasars whose positions are estimated to be accurateat the 0.25 mas level. We use our results together with results ofprevious observations to obtain proper-motion estimates for these stars.The average proper-motion uncertainties are 1.1 mas yr-1 inμαcosδ and 1.2 mas yr-1 inμδ, comparable to the Hipparcos values.
| The `solar model problem' solved by the abundance of neon in nearby stars
The interior structure of the Sun can be studied with great accuracyusing observations of its oscillations, similar to seismology of theEarth. Precise agreement between helioseismological measurements andpredictions of theoretical solar models has been a triumph of modernastrophysics. A recent downward revision by 25-35 per cent of the solarabundances of light elements such as C, N, O and Ne (ref. 2) has,however, broken this accordance: models adopting the new abundancesincorrectly predict the depth of the convection zone, the depth profilesof sound speed and density, and the helium abundance. The discrepanciesare far beyond the uncertainties in either the data or the modelpredictions. Here we report neon-to-oxygen ratios measured in a sampleof nearby solar-like stars, using their X-ray spectra. The abundanceratios are all very similar and substantially larger than the recentlyrevised solar value. The neon abundance in the Sun is quite poorlydetermined. If the Ne/O abundance in these stars is adopted for the Sun,the models are brought back into agreement with helioseismologymeasurements.
| Spurious `active longitudes' in parametric models of heavily spotted eclipsing binaries
In this paper, the size distributions of starspots extrapolated from thecase of the Sun are modelled on the eclipsing binary SV Cam tosynthesize images of stellar photospheres with high spot fillingfactors. These spot distributions pepper the primary's surface withspots, many of which are below the resolution capabilities ofeclipse-mapping and Doppler-imaging techniques. The light curvesresulting from these modelled distributions are used to determine thelimitations of image reconstruction from photometric data. Surfacebrightness distributions reconstructed from these light curves showdistinctive spots on the primary star at its quadrature points. It isconcluded that two-spot modelling or chi-squared minimization techniquesare more susceptible to spurious structures being generated bysystematic errors, arising from incorrect assumptions about photosphericsurface brightness, than simple Fourier analysis of the light curves.
| Search for activity-induced variability in AR Lac, KT Peg, KZ And, II Peg and EI Eri in autumn 1997
We report on a search for activity-induced variability in the SB2systems AR Lac, KT Peg and KZ And as well as in the SB1 systems II Pegand EI Eri in the September 1997 season. The study of Hα lineprofiles, Na I 5889Angstrom doublet, Ca II IRT lines and TiO7055Angstrom absorption band is presented. The spot properties wereestimated for the systems AR Lac and II Peg. The former possesses spotsat intermediate stellar latitudes on the primary component while thelatter shows these surface inhomogeneities at low latitudes. In bothcases, the spots cooler than the surrounding photosphere were revealed.Significant variability in the hydrogen Hα line profile wasobserved on II Peg and EI Eri, and the evidence of a rotationalmodulation was found in KT Peg and II Peg.
| Stellar Coronal Abundances at Intermediate-Activity Levels: ξ UMa
We present an analysis of Chandra High Energy Transmission Grating(HETG) spectra of the corona of the critical intermediate-activity,quadruple stellar system ξ UMa. Using the maximum A/B subsystemseparation in 2001, we attempted to resolve for the first time in X-raysthe two components using the HETG Medium Energy Grating (MEG). We foundthe Aa component of the system to be at least 2 orders of magnitudefainter than the Bb component. We used the Markov Chain Monte Carlomethod to reconstruct differential emission measures (DEMs) andabundances and discuss first ionization potential (FIP) issues. Thereconstructed DEMs showed two pronounced peaks at logT~6.5 and logT~7.0K, respectively, similar to brighter solar active regions. A plot ofcoronal abundances, with respect to stellar photospheric, versus FIPreveals that the FIP behavior of ξ UMa is intermediate between lessand more active stars, commensurate with its X-ray luminosity. Theabundances of Mg, Fe, and Si are just subsolar, with Fe having thelowest abundance of these. The low-FIP elements Na and Al have similarabundances, and they are both higher than the Mg, Fe, and Si group. Theabundances show a minimum at S, rising gradually to Ne, which is higherthan photospheric, as has been seen by other authors on similar stars.Some of the FIP behavior seen here is predicted by the new ponderomotiveforce model of Laming.
| Coronal Evolution of the Sun in Time: High-Resolution X-Ray Spectroscopy of Solar Analogs with Different Ages
We investigate the long-term evolution of X-ray coronae of solar analogsbased on high-resolution X-ray spectroscopy and photometry withXMM-Newton. Six nearby main-sequence G stars with ages between ~0.1 and~1.6 Gyr and rotation periods between ~1 and 12.4 days have beenobserved. We use the X-ray spectra to derive coronal element abundancesof C, N, O, Ne, Mg, Si, S, and Fe and the coronal emission measuredistribution (EMD). We find that the abundances change from an inversefirst ionization potential (FIP) distribution in stars with ages around0.1 Gyr to a solar-type FIP distribution in stars at ages of 0.3 Gyr andbeyond. This transformation is coincident with a steep decline ofnonthermal radio emission. The results are in qualitative agreement witha simple model in which the stream of electrons in magnetic fieldssuppresses diffusion of low-FIP ions from the chromosphere into thecorona. The coronal emission measure distributions show shapescharacterized by power laws on each side of the EMD peak. The lattershifts from temperatures of about 10 MK in the most rapidly rotating,young stars to temperatures around 4 MK in the oldest target consideredhere. The power-law index on the cooler side of the EMD exceeds expectedslopes for static loops, with typical values being 1.5-3. We interpretthis slope with a model in which the coronal emission is due to asuperposition of stochastically occurring flares, with an occurrencerate that is distributed in radiated energy E as a power law,dN/dE~E-α, as previously found for solar and stellarflares. We obtain the relevant power-law index α from the slope ofthe high-temperature tail of the EMD. Our EMDs indicate α~2.2-2.8,in excellent agreement with values previously derived from light curvesof magnetically active stars. Modulation with timescales reminiscent offlares is found in the light curves of all our targets. Several strongflares are also observed. We use our α-values to simulate lightcurves and compare them with the observed light curves. We thus derivethe range of flare energies required to explain the light-curvemodulation. More active stars require a larger range of flare energiesthan less active stars within the framework of this simplistic model. Inan overall scenario, we propose that flaring activity plays a largerrole in more active stars. In this model, the higher flare rate isresponsible both for the higher average coronal temperature and the highcoronal X-ray luminosity, two parameters that are indeed found to becorrelated.
| The Coronae of AB Doradus and V471 Tauri: Primordial Angular Momentum versus Tidal Spin-up
The zero-age main-sequence star AB Dor and the K dwarf component of theV471 Tau close binary have essentially identical rotation rates andspectral types. An analysis of their high-resolution Chandra X-rayspectra reveals remarkably similar coronal characteristics in terms ofboth temperature structure and element abundances. Both stars showdepletions of low first ionization potential (FIP) elements by factorsof ~3, with higher FIP elements showing more mild depletions. Noevidence for enhancements of very low FIP (<7 eV) elements, such asNa, Al, and Ca, as compared to other low-FIP elements, was found. Theabundance anomaly pattern for AB Dor and V471 Tau is similar to,although less extreme than, the abundance anomalies exhibited by activeRS CVn-type binaries. While we find statistically significant structurein the underlying differential emission measure distributions of thesestars over narrow temperature intervals, this structure is stronglydependent on the lines used in the analysis and is probably spurious. Onthe basis of their X-ray similarities, we conclude that the exactevolutionary state of a star has little effect on coronalcharacteristics and that the parameters that dominate coronal structureand composition are simply the rotation rate and spectral type.
| Stellar dynamos - theoretical aspects
I review some dynamo models and constraints for stars other than theSun. The models include mean-field dynamo models for RS Cvn binarysystems and global magnetoconvection simulations for supergiants and forfully convective stars. In the latter case, we find generation ofmixed-parity large-scale magnetic fields once the dynamo reachessaturation. In addition to the cool, narrow downdrafts known from localsimulations, these models also show a hot convective plume stretchingfrom the centre towards the surface.
| Spectroscopic and Photometric Observations of the Short-Period RS CVn-Type Star UV Piscium
High-resolution spectroscopic observations around the Hα line andlong-term BVRI photometry of the eclipsing short-period RS CVn star UVPsc are presented. The simultaneous solution of our radial velocitycurves and light curves yields the following values for the globalparameters of the components: M1=1.1 Msolar,M2=0.81 Msolar, R1=1.14Rsolar, and R2=0.85 Rsolar. Themeasured rotational broadening of the spectral lines corresponds toequatorial velocities V1=68.3 and V2=53.3 kms-1. Our spectral data reveal high activity of the twostellar components and very fast variability of the Hα line in thecenter of the primary eclipse. Modeling our photometric data showstrends in the starspot behavior. The trend toward active longitude beltscan have occasional exceptions. The observed secular luminosity decreasein 1999 may signal the onset of a new magnetic activity cycle.
| Localizing plages on BO Mic. Rapid variability and rotational modulation of stellar Ca H&K core emission
We have obtained a densely sampled time series of Ca ii H&K lineprofiles of the ultrafast rotating K-dwarf star BOMic. Taken at high resolution, the spectra reveal pronouncedvariations of the emission core profiles. We interpret these variationsas signs of concentrated chromospherically active regions, in analogy tosolar plages. We further interpret the variations as partly due to therapid growth and decay of plages, while other variations appear to becaused by plages moved over the visible stellar disk by rotation. Theequivalent width of the Ca K core emission changes approximately inanti-phase to the photospheric brightness, suggesting an association ofthe chromospheric plage regions with pronounced dark photospheric spots.We believe that further analysis of the presented spectral time serieswill lead to a chromospheric Doppler image of BO mic.
| Hα observations of the star FK Com
High-resolution spectroscopic observations around the Hα line ofFK Com are presented. The analysis of the spectrareveals that: (a) the intensity of the R peak of the mean Hαprofile is larger than that of the V peak; (b) the intensities of the Vand R emission peaks changes almost in anti-phase; (c) the ratio V/Rvaries in phase with V; (d) the total Hα emission has a maximum atphase 0.75. The similarity of the Hα profile of FK Com with thoseof the disk-like stars leads us to the conclusion that its broadtwo-peaked emission Hα line originates from an extended disk. Itis supposed that the disk produces the mean Hα line. Thevariability of the Hα profile is explained by the presence ofadditional sources of absorption and emission at diametrically oppositeregions of the disk. The asymmetry of the mean Hα profile requiresthe disk to be half-illuminated. It is supposed that a source ofillumination is a low-mass hot secondary orbiting the disk. A model ofthe Hα emitting configuration of FK Com is proposed.
| Mg II chromospheric radiative loss rates in cool active and quiet stars
The Mg II k emission line is a good indicator of the level ofchromospheric activity in late-type stars. We investigate the dependenceof this activity indicator on fundamental stellar parameters. To thispurpose we use IUE observations of the Mg II k line in 225 late-typestars of luminosity classes I-V, with different levels of chromosphericactivity. We first re-analyse the relation between Mg II k lineluminosity and stellar absolute magnitude, performing linear fits to thepoints. The ratio of Mg II surface flux to total surface flux is foundto be independent of stellar luminosity for evolved stars and toincrease with decreasing luminosity for dwarfs. We also analyse the MgII k line surface flux-metallicity connection. The Mg II k emissionlevel turns out to be not dependent on metallicity. Finally, the Mg II kline surface flux-temperature relation is investigated by treatingseparately, for the first time, a large sample of very active and normalstars. The stellar surface fluxes in the k line of normal stars arefound to be strongly dependent on the temperature and slightly dependenton the gravity, thus confirming the validity of recently proposedmodels. In contrast, data relative to RS CVn binaries and BY Dra stars,which show very strong chromospheric activity, are not justified in theframework of a description based only on acoustic waves and uniformlydistributed magnetic flux tubes so that they require more detailedmodels.
| Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters
The availability of the Hipparcos Catalogue has triggered many kinematicand dynamical studies of the solar neighbourhood. Nevertheless, thosestudies generally lacked the third component of the space velocities,i.e., the radial velocities. This work presents the kinematic analysisof 5952 K and 739 M giants in the solar neighbourhood which includes forthe first time radial velocity data from a large survey performed withthe CORAVEL spectrovelocimeter. It also uses proper motions from theTycho-2 catalogue, which are expected to be more accurate than theHipparcos ones. An important by-product of this study is the observedfraction of only 5.7% of spectroscopic binaries among M giants ascompared to 13.7% for K giants. After excluding the binaries for whichno center-of-mass velocity could be estimated, 5311 K and 719 M giantsremain in the final sample. The UV-plane constructed from these datafor the stars with precise parallaxes (σπ/π≤20%) reveals a rich small-scale structure, with several clumpscorresponding to the Hercules stream, the Sirius moving group, and theHyades and Pleiades superclusters. A maximum-likelihood method, based ona Bayesian approach, has been applied to the data, in order to make fulluse of all the available stars (not only those with precise parallaxes)and to derive the kinematic properties of these subgroups. Isochrones inthe Hertzsprung-Russell diagram reveal a very wide range of ages forstars belonging to these groups. These groups are most probably relatedto the dynamical perturbation by transient spiral waves (as recentlymodelled by De Simone et al. \cite{Simone2004}) rather than to clusterremnants. A possible explanation for the presence of younggroup/clusters in the same area of the UV-plane is that they have beenput there by the spiral wave associated with their formation, while thekinematics of the older stars of our sample has also been disturbed bythe same wave. The emerging picture is thus one of dynamical streamspervading the solar neighbourhood and travelling in the Galaxy withsimilar space velocities. The term dynamical stream is more appropriatethan the traditional term supercluster since it involves stars ofdifferent ages, not born at the same place nor at the same time. Theposition of those streams in the UV-plane is responsible for the vertexdeviation of 16.2o ± 5.6o for the wholesample. Our study suggests that the vertex deviation for youngerpopulations could have the same dynamical origin. The underlyingvelocity ellipsoid, extracted by the maximum-likelihood method afterremoval of the streams, is not centered on the value commonly acceptedfor the radial antisolar motion: it is centered on < U > =-2.78±1.07 km s-1. However, the full data set(including the various streams) does yield the usual value for theradial solar motion, when properly accounting for the biases inherent tothis kind of analysis (namely, < U > = -10.25±0.15 kms-1). This discrepancy clearly raises the essential questionof how to derive the solar motion in the presence of dynamicalperturbations altering the kinematics of the solar neighbourhood: doesthere exist in the solar neighbourhood a subset of stars having no netradial motion which can be used as a reference against which to measurethe solar motion?Based on observations performed at the Swiss 1m-telescope at OHP,France, and on data from the ESA Hipparcos astrometry satellite.Full Table \ref{taba1} is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/430/165}
| Orbital period variation in close binaries from radial velocity data and magnetic activity cycles. II. HR 1099
We studied orbital period changes in the non-eclipsing close binary HR1099, one of the most bright members of the RS CVn class of magneticallyactive binary systems. Intermediate-resolution optical spectroscopy andIUE archive spectra were used to build radial-velocity curves yieldingepochs of superior conjunction with an accuracy of 0.01 days. The finaldatabase ranged from 1976 to 2002 and allowed us a better assessment ofthe amplitude of the orbital period variation and its timescale. On thebasis of such results, the mechanisms proposed to explain the observedperiod change were briefly discussed giving further support to thepossible connection between the orbital period modulation and the changeof the gravitational quadrupole moment of the K1 subgiant component, inthe framework of the model elaborated by Lanza et al. (\cite{Lanza98}).Based on IUE archive data and observations collected at CataniaAstrophysical Observatory, Italy.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/429/309
| Kinematics of chromospherically active binaries and evidence of an orbital period decrease in binary evolution
The kinematics of 237 chromospherically active binaries (CABs) werestudied. The sample is heterogeneous with different orbits andphysically different components from F to M spectral-type main-sequencestars to G and K giants and supergiants. The computed U, V, W spacevelocities indicate that the sample is also heterogeneous in velocityspace. That is, both kinematically younger and older systems exist amongthe non-evolved main sequence and the evolved binaries containing giantsand subgiants. The kinematically young (0.95 Gyr) subsample (N= 95),which is formed according to the kinematical criteria of moving groups,was compared with the rest (N= 142) of the sample (3.86 Gyr) toinvestigate any observational clues of binary evolution. Comparing theorbital period histograms between the younger and older subsamples,evidence was found supporting the finding of Demircan that the CABs losemass (and angular momentum) and evolve towards shorter orbital periods.The evidence of mass loss is noticeable on the histograms of the totalmass (Mh+Mc), which is compared between theyounger (only N= 53 systems available) and older subsamples (only N= 66systems available). The orbital period decrease during binary evolutionis found to be clearly indicated by the kinematical ages of 6.69, 5.19and 3.02 Gyr which were found in the subsamples according to the periodranges of logP<= 0.8, 0.8 < logP<= 1.7 and 1.7 < logP<=3, respectively, among the binaries in the older subsample.
| CCD Times of Minima of Selected Eclipsing Binaries
682 CCD minima observations of 259 eclipsing binaries made mainly byauthor are presented. The observed stars were chosen mainly fromcatalogue BRKA of observing programme of BRNO-Variable Star Section ofCAS.
| Variations in the Solar Luminosity, Radius, and Quadrupole Moment as Effects of a Large-Scale Dynamo in the Solar Convection Zone
The effect of large-scale magnetic fields generated by the solar dynamoon the irradiance of the Sun and stratification of the solar convectionzone is studied using a numerical model of a spherical axisymmetricdynamo. This model provides a joint description of the generation oflarge-scale magnetic fields, differential rotation, and convective heattransfer taking into account energy transformations associated with thelarge-scale magnetic fields, as well as the stratification of theconvection zone. The model further develops a previously suggestedself-consistent approach to analyzing solar luminosity variations, basedon the conservation of the energy of the large-scale magnetic fields andturbulent flows. The results indicate that the increase in the solarluminosity near the maximum of the cycle is mainly due to thedissipation of the energy of magnetic fields escaping to above thephotosphere, with the partial conversion of this energy into radiation.In addition, near-photospheric magnetic fields strongly affect thelatitudinal nonuniformity of the cyclic variations in the radiativeflux. The large-scale magnetic field also influences the hydrostaticequilibrium of the convection zone and gives rise to 11-year variationsin the sound speed with a relative amplitude of 10-3. The cyclicmagnetic activity generates oscillations in the quadrupole moment withan amplitude of 4.5 × 10-9(GM ȯ/R ȯ). According to ourestimates, the variations in the solar radius are very small, 10-6R ȯ. Our numerical model is used to estimate the variations in theorbital periods of close binaries whose primaries have the same spectralclass as the Sun. Modulation of the centrifugal force by torsionaloscillations can provide a plausible explanation for variations in theorbital periods of the companion stars in these systems.
| The Burst and Transient Source Experiment (BATSE) Earth Occultation Catalog of Low-Energy Gamma-Ray Sources
The Burst and Transient Source Experiment (BATSE), aboard the ComptonGamma Ray Observatory (CGRO), provided a record of the low-energygamma-ray sky (~20-1000 keV) between 1991 April and 2000 May (9.1 yr).BATSE monitored the high-energy sky using the Earth occultationtechnique (EOT) for point sources whose emission extended for times onthe order of the CGRO orbital period (~92 min) or greater. Using the EOTto extract flux information, a catalog of sources using data from theBATSE Large Area Detectors has been prepared. The first part of thecatalog consists of results from the all-sky monitoring of 58 sources,mostly Galactic, with intrinsic variability on timescales of hours toyears. For these sources, we have included tables of flux and spectraldata, and outburst times for transients. Light curves (or fluxhistories) have been placed on the World Wide Web.We then performed a deep-sampling of these 58 objects, plus a selectionof 121 more objects, combining data from the entire 9.1 yr BATSE dataset. Source types considered were primarily accreting binaries, but asmall number of representative active galaxies, X-ray-emitting stars,and supernova remnants were also included. The sample represents acompilation of sources monitored and/or discovered with BATSE and otherhigh-energy instruments between 1991 and 2000, known sources taken fromthe HEAO 1 A-4 and Macomb & Gehrels catalogs. The deep sampleresults include definite detections of 83 objects and possibledetections of 36 additional objects. The definite detections spannedthree classes of sources: accreting black hole and neutron starbinaries, active galaxies, and supernova remnants. The average fluxesmeasured for the fourth class, the X-ray emitting stars, were below theconfidence limit for definite detection.Flux data for the deep sample are presented in four energy bands: 20-40,40-70, 70-160, and 160-430 keV. The limiting average flux level (9.1 yr)for the sample varies from 3.5 to 20 mcrab (5 σ) between 20 and430 keV, depending on systematic error, which in turn is primarilydependent on the sky location.To strengthen the credibility of detection of weaker sources (~5-25mcrab), we generated Earth occultation images, searched for periodicbehavior using FFT and epoch folding methods, and critically evaluatedthe energy-dependent emission in the four flux bands. The deep sampleresults are intended for guidance in performing future all-sky surveysor pointed observations in the hard X-ray and low-energy gamma-ray band,as well as more detailed studies with the BATSE EOT.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| The Density of Coronal Plasma in Active Stellar Coronae
We have analyzed high-resolution X-ray spectra of a sample of 22 activestars observed with the High Energy Transmission Grating Spectrometer onChandra in order to investigate their coronal plasma density. Densitieswere investigated using the lines of the He-like ions O VII, Mg XI, andSi XIII. Si XIII lines in all stars of the sample are compatible withthe low-density limit (i.e., ne<~1013cm-3), casting some doubt on results based on lowerresolution Extreme Ultraviolet Explorer (EUVE) spectra finding densitiesne>1013 cm-3. Mg XI lines betray thepresence of high plasma densities up to a few times 1012cm-3 for most of the sources with higher X-ray luminosity(>~1030 ergs s-1) stars with higherLX and LX/Lbol tend to have higherdensities at high temperatures. Ratios of O VII lines yield much lowerdensities of a few times 1010 cm-3, indicatingthat the ``hot'' and ``cool'' plasma resides in physically differentstructures. In the cases of EV Lac, HD 223460, Canopus, μ Vel, TYPyx, and IM Peg, our results represent the first spectroscopic estimatesof coronal density. No trends in density-sensitive line ratios withstellar parameters effective temperature and surface gravity were found,indicating that plasma densities are remarkably similar for stars withpressure scale heights differing by up to 3 orders of magnitude. Ourfindings imply remarkably compact coronal structures, especially for thehotter (~7 MK) plasma emitting the Mg XI lines characterized by thecoronal surface filling factor, fMgXI, ranging from10-4 to 10-1, while we find fOVIIvalues from a few times 10-3 up to ~1 for the cooler (~2 MK)plasma emitting the O VII lines. We find that fOVIIapproaches unity at the same stellar surface X-ray flux level ascharacterizes solar active regions, suggesting that these stars becomecompletely covered by active regions. At the same surface flux level,fMgXI is seen to increase more sharply with increasingsurface flux. These results appear to support earlier suggestions thathot 107 K plasma in active coronae arises from flaringactivity and that this flaring activity increases markedly once thestellar surface becomes covered with active regions. Comparison of ourmeasured line fluxes with theoretical models suggests that significantresidual model inaccuracies might be present and, in particular, thatcascade contributions to forbidden and intercombination lines resultingfrom dielectronic recombination might be to blame.
| Detection of X-Ray Resonance Scattering in Active Stellar Coronae
An analysis of Lyman series lines arising from hydrogen-like oxygen andneon ions in the coronae of the active RS CVn-type binaries II Peg andIM Peg, observed using the Chandra High Resolution Transmission GratingSpectrograph, shows significant decrements in the Lyα/Lyβratios as compared with theoretical predictions and with the same ratiosobserved in similar active binaries. We interpret these decrements interms of resonance scattering of line photons out of the line of sight;these observations present the first strong evidence of this effect inactive stellar coronae. The net line photon loss implies a nonuniformand asymmetric surface distribution of emitting structures on thesestars. Escape probability arguments, together with the observed lineratios and estimates of the emitting plasma density, imply typicalline-of-sight sizes of the coronal structures that dominate the X-rayemission of 1010 cm at temperatures of 3×106K and of 108 cm at 107 K. These sizes are an orderof magnitude larger than predicted by simple quasi-static coronal loopsmodels but are still very small compared to the several 1011cm radii of the underlying stars.
| Positional Uncertainties and Detection Limits of ROSAT X-Ray Sources
This study addresses, primarily, positional uncertainties of sourcesimaged by the X-ray cameras on the Röntgensatellit (ROSAT). MonteCarlo simulations are exploited to estimate the precision of centroidingpoint-source ``photon clouds'' in ROSAT event lists. Measurements ofbright coronal stars demonstrate that the design specification 6"accuracy of a ROSAT pointing center was achieved in practice.Furthermore, the accuracy often can be improved to 2" or better bymatching serendipitous X-ray sources to an optical catalog like the USNaval Observatory A2. Even a low signal-to-noise detection usually canbe localized well enough to obtain a unique identification with acataloged object or to establish that no bright optical counterpart ispresent. Secondarily, and in a more general context, the study addressesdetection significance and flux limits in the sparse counting regime.Building on previous work by Gehrels and Kraft, Burrows, & Nousek,scaling laws are developed to estimate detection thresholds and lowerand upper bounds on possible source fluxes. Although the specificapplication of ROSAT was in mind, the flux limit approximations apply toother situations governed by Poisson statistics; including ultravioletand X-ray spectra with low source counts and nonnegligible backgrounds.
| Stellar activity and the long-term use of robotic telescopes
A number of automated and robotic telescopes are nowadays devoted to thesystematic monitoring of magnetically active stars and binary systems atseveral astronomical institutions, all over the world, and their numberis steadily increasing. Standard equipments include wide- andnarrow-band photometers and, more recently, spectroscopic capabilities.The long-term time series that those telescopes are providing turn outto be of paramount importance in order to significantly progress in ourunderstanding of solar-like stellar activity of magnetic origin, thatseemingly affect most of late-type dwarfs and subgiants. Our principalaim is to illustrate which key parameters, that can be derived from suchlong-term time series, determine the appearance and evolution of stellaractivity phenomena in different astrophysical environments other thansolar, and their role in determining the physical characteristics ofstarspots, their surface distribution, filling factor, migration inlatitude and longitude, and evolution in time. By using spots as tracersof stellar rotation, reliable data on stellar differential rotation, theprime motor of magnetic activity, can be derived. Moreover, the activitycycle is the additional fundamental parameter that can be provided bylong-term time series. In order to properly address the study of stellaractivity, an internationally coordinated network of 1-2 m class robotictelescopes dedicated to multi-wavelength systematic observations shouldbe established.
| Empirically Constrained Color-Temperature Relations. II. uvby
A new grid of theoretical color indices for the Strömgren uvbyphotometric system has been derived from MARCS model atmospheres and SSGsynthetic spectra for cool dwarf and giant stars having-3.0<=[Fe/H]<=+0.5 and 3000<=Teff<=8000 K. Atwarmer temperatures (i.e., 8000-2.0. To overcome thisproblem, the theoretical indices at intermediate and high metallicitieshave been corrected using a set of color calibrations based on fieldstars having well-determined distances from Hipparcos, accurateTeff estimates from the infrared flux method, andspectroscopic [Fe/H] values. In contrast with Paper I, star clustersplayed only a minor role in this analysis in that they provided asupplementary constraint on the color corrections for cool dwarf starswith Teff<=5500 K. They were mainly used to test thecolor-Teff relations and, encouragingly, isochrones thatemploy the transformations derived in this study are able to reproducethe observed CMDs (involving u-v, v-b, and b-y colors) for a number ofopen and globular clusters (including M67, the Hyades, and 47 Tuc)rather well. Moreover, our interpretations of such data are verysimilar, if not identical, with those given in Paper I from aconsideration of BV(RI)C observations for the sameclusters-which provides a compelling argument in support of thecolor-Teff relations that are reported in both studies. Inthe present investigation, we have also analyzed the observedStrömgren photometry for the classic Population II subdwarfs,compared our ``final'' (b-y)-Teff relationship with thosederived empirically in a number of recent studies and examined in somedetail the dependence of the m1 index on [Fe/H].Based, in part, on observations made with the Nordic Optical Telescope,operated jointly on the island of La Palma by Denmark, Finland, Iceland,Norway, and Sweden, in the Spanish Observatorio del Roque de losMuchachos of the Instituto de Astrofisica de Canarias.Based, in part, on observations obtained with the Danish 1.54 mtelescope at the European Southern Observatory, La Silla, Chile.
| X-ray astronomy of stellar coronae
X-ray emission from stars in the cool half of the Hertzsprung-Russelldiagram is generally attributed to the presence of a magnetic coronathat contains plasma at temperatures exceeding 1 million K. Coronae areubiquitous among these stars, yet many fundamental mechanisms operatingin their magnetic fields still elude an interpretation through adetailed physical description. Stellar X-ray astronomy is thereforecontributing toward a deeper understanding of the generation of magneticfields in magnetohydrodynamic dynamos, the release of energy in tenuousastrophysical plasmas through various plasma-physical processes, and theinteractions of high-energy radiation with the stellar environment.Stellar X-ray emission also provides important diagnostics to study thestructure and evolution of stellar magnetic fields from the first daysof a protostellar life to the latest stages of stellar evolution amonggiants and supergiants. The discipline of stellar coronal X-rayastronomy has now reached a level of sophistication that makes tests ofadvanced theories in stellar physics possible. This development is basedon the rapidly advancing instrumental possibilities that today allow usto obtain images with sub-arcsecond resolution and spectra withresolving powers exceeding 1000. High-resolution X-ray spectroscopy has,in fact, opened new windows into astrophysical sources, and has played afundamental role in coronal research.
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| Takýmyýldýz: | Kertenkele |
| Sað Açýklýk: | 22h08m41.00s |
| Yükselim: | +45°44'31.0" |
| Görünürdeki Parlaklýk: | 6.11 |
| Uzaklýk: | 42.034 parsek |
| özdevim Sað Açýklýk: | -51.8 |
| özdevim Yükselim: | 49.1 |
| B-T magnitude: | 7.061 |
| V-T magnitude: | 6.226 |
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